Nestar 3 In 1 Local Network Links PCS, Electronics Dec 29 1982

Electronics

3-in-1 local network
links personal computers

Arcnet, Ethernet, and Cluster/One techniques team up
to expand small-net scheme with new capabilities

by W. Pearson, G. M. Ellis, J. D. Whitnell, C. W. Payne, and S. Dillon, Nestar Systems Inc., Palo Alto, Calit.

C Instead of settling for just one local-network tech-
nique, a new system integrates elements of three different
networks: Arcnet, Ethernet, and Cluster/One. The first
provides the basic token-passing network, the second
adds the internetworking software, and the third supplies
the software of a net based on personal computers.

Three systems are better than one in this case because
of the need for a smooth transition from the small,
proprietary networks of today to larger networks that
can keep up with the growth of personal computing in
business and other professional applications. The new
PLAN (personal local-area network) 4000 system is there-
fore designed to support around 10 times as much data
traffic as most low-cost, personal-computer nets.

Cost-effective net

PLAN can expand to any size that may be needed, and
it will support the development of corporate-wide net-
works containing a variety of network types and comput-
ing resources. Yet it adds little to the cost of personal
computers and starts out with software already proven
on such machines.

The PLAN system currently links Apple II, Apple ITI
and IBM Personal Computers into networks containing
work stations and file, print, and communications servers

PRINT SERVER

FILE-

TRANSFER
SERVER

IBM-3270
EMULATOR

TO TO REMOTE

REMOTE
NETWORK

MAINFRAME
COMPUTER

(Fig. 1). As a server-based net, the intelligence is distrib-
uted among the various work stations and similar com-
puters (servers) performing such functions as file and
printer management for the net. Thus there is no central
host computer, as in some other networks.

Most of the server programs and other software for
data sharing, remote communications, and electronic
mail previously ran on Apple computers in Cluster/One
networks. It now runs on Personal Computer operating
systems as well, so that all three computer types can
operate in a single network.

Revised servers

There are are also a new file-server subsystem and new
IBM-3270 and -3780 terminal emulators. The file server
expands network-wide, shared-data capacity to more
than 500 megabytes per server and has multiple ports for
directly connecting other facilities. The original gateway-
server program now links PLAN segments together into
networks larger than the standard Arcnet limit of 255
nodes and approximately 4 miles. It also interconnects
Cluster/One networks with PLAN nets and will support
interconnection of other networks, such as Ethernet.
Cluster/One applications can be upgraded to PLAN net-
works without software modification by adding the new

DEDICATED
SYSTEM
NETWORK
FILE

SERVER

C] LINE-ISOLATION DEVICE
C] WORK STATION (PERSONAL COMPUTER)

1. Star-burst. Clusters of Apple and IBM Personal Computer work stations and servers are linked into subnetworks and connected to a high-per-
formance file server. All connections are made through line-isolation devices that also identify any malfunctioning nodes.

THREE LOCAL NETWORKS COMPARED

Ethernet

Sos
cable — | cable

token =] carrier-sense

passing multiple-access
: with collision

detection

-| $1,000 and up

interface hardware and software enhancements.

In effect, a valuable house that has taken more than
three years to build—the Cluster/One software—has
been moved to a new foundation that provides more
facilities. Such upgrades are needed to support the
growth of resource-sharing communities in business and
other professional applications of personal computers.
The server-based type of system has become popular in
resource-sharing applications during the last few years,
but the proprietary designs necessary in the past to re-
duce network costs have generally limited them to small
networks. The work stations of the Cluster/One system,
for example, execute proprietary data-link protocols in
software and support only one vendor’s computers (Ap-
ple). The complex software interface limits data rate and
expandability (see table, above).

In the PLAN adaptation of the Arcnet system, the
network interface is a custom set of three integrated
circuits supported by Ethernet software handling inter-
network data communications and transport functions
The Ics handle link functions; for instance, work stations
use the Arcnet broadcast mode (transmission to all

carrier-sense
multiple-access
with collision
avoidance

Station connection
costs

READ/WRITE
REQUEST
WAIT

CONTROL LINES

DATA/ADDRESS

INTERFACE
LOGIC

nodes) to locate a file server containing the programs
needed to activate user operating systems.

The chip set, located on the work-station and server
computers’ network-interface cards, includes a high-
speed custom MOS controller that implements the net-
work algorithm in microcode at high speed (Fig. 2).
Because custom chips are used, the computer-connection
cost is below $700—a cost that includes the per-comput-
er share of the line-interface hardware. The coaxial cable
is also inexpensive—RG-62 with twist-on connectors.
Compatible with IBM-3270 terminal systems and stan-
dard in Arcnet systems, it is already installed in more
than a million offices.

This basic network setup corresponds to the first four
layers of the International Standards Organization’s open
systems interconnection reference model for multilayer
computer-network architectures (Fig. 3). The cabling,
line-isolation devices and interface ICs form the physical
and data-link layers, which handle data transmission,
arbitration (in token passing, each node must wait for its
turn to transmit), and intranetwork addressing. The net-
work and transport software performs such functions as
internetwork addressing and routing, interprocess com-
munications and transmission error control.

Logical choices

Arcnet and Ethernet implementations of these layers
were chosen because they are becoming de facto stan-
dards for high-performance networks. A transition to
such standards is highly desirable in order to eliminate
current limitations on multivendor support for low-cost
networks and to expedite the integration of these systems
with other nets.

Arcnet interface specifications were made available
about a year ago by Datapoint Corp., San Antonio, Tex-
as, which also helped Standard Microsystems Corp.,
Hauppauge, N. Y., develop an MOS large-scale IC for
general use (the custom chip set devised for the PLAN net
was derived from this design). At about the same time,
Xerox Corp., Stamford, Conn., published the high-level,
Xerox Network System (XNS) internet transport proto-

7 a TIMING
MICROCODED

NETWORK |
CONTROLLER

TRANSMIT

AND
- RECEIVE
SERIAL 7
DATA

2-K-BYTE
DUAL-PORT
BUFFER

; NETWORK-
HOST E INTERFACE

CARD

BUFFER CONTROL

2. Network interface. Custom chip set (color) on network-interface card implements the network algorithm and interfaces the cable. The three
chips form an Arcnet interface that is supported by Ethernet high-level protocols executed in software by the computer.

LAYERS OF
OPEN-SYSTEMS
INTERCONNECTION
REFERENCE MODEL

PLAN SYSTEM
FUNCTIONS

NESTAR
NETWORK
SOFTWARE

XEROX
ETHERNET

© DATAPOINT
© ARCNET

_PHYSICAL LINK

3. Stacked up. Overall system design parallels multilayer architecture
recommended by the International Standards Organization. An Arc-
net-compatible interface forms the bottom layers, Xerox internet
transport protocols the next two, and Nestar software the rest.

cols—the ones that are used in the PLAN system.

The PLAN system employs Arcnet, rather than Ether-
net data-link protocols, because the former’s token-pass-
ing technique is simpler and more efficient, reduces inter-
face logic costs, and provides a topology well suited to
the way personal-computer networks are organized.
However, the XNS internet transport protocols were cho-
sen for the network and transport software layers be-
cause they are the best-defined and most powerful in the
public domain and correspond to the proposed ISO net-
work and transport architectures.

These Xerox protocols are the key to the overall up-
grade of Cluster/One because they are essentially inde-
pendent of the lower-level data-link protocols and the

type of application-oriented services, such as terminal.

emulation and server operation, provided by the higher
layers. In effect, they allow the existing software to be
reconnected to the underlying utilities with little change.
Another major advantage is that other upper-layer soft-
ware systems written to the XNS protocols can be added
to the PLAN system as well. Further, this choice will
simplify development of PLAN-Ethernet gateways, an im-
portant consideration for users who envision hybrid sys-
tems. The network and transport functions are utilized
automatically by the file, print, and communication serv-
ers, so the operating details can be ignored by users.
However, PLAN software library modules make them ac-
cessible for development by equipment manufacturers
and users of specialized servers and custom station-to-
station communication techniques.

Software choices

The rest of the network’s system software extends up
to the ISO application layer, which consists of servers,
electronic-mail services, and the like. PLAN application

software includes such general-purpose programs as-

Multi-Calc, a network version of the VisiCalc spread-
sheet program, and the NPL (for nonprocedural language)
data-based management system from Desktop Software.
What’s more, other applications programs are available
from Nestar-supported software companies.

Although customized, the PLAN interface is hardware-
compatible with the interfaces in Datapoint’s minicom-

puter-based Arcnet networks and has the same perfor-
mance. In fact, performance evaluations published by
Datapoint demonstrate that even in very large networks,
which require many token-passing operations, several
hundred messages per second can be transmitted success-
fully [Electronics, Sept. 8, 1982, p. 158].

Most small local nets depend for cost reduction on
simplified, low-speed versions of carrier-sense, multiple-
access (CSMA) data-link protocols. Cluster/One employs
a software form of CSMA/CD (CSMA with collision detec-
tion) known as CSMA/CA (collision avoidance). The
changeover from such protocols to token passing can
produce a greater increase in overall network perfor-
mance than is indicated by the increase in raw data rate
(to 2.5 megabits a second), because of the efficiency of
the Arcnet protocols in heavily populated networks.

Matching Ethernet

Moreover, token passing compares favorably with Eth-
ernet CSMA/CD hardware-based network control, despite
its relatively low cost. One major advantage is that the
token (the logical invitation to transmit a message) goes
from node to node in a specified sequence. This arrange-
ment guarantees that a node can transmit a message
within a particular time interval, so a system designer
can calculate a precise, worst-case transmission delay.
Also, before a node starts actual data transmission, it can
check the receiving node to ensure that buffer space will
be available for its message, which prevents wasting traf-
fic capacity on useless transmissions.

The token passes between nodes in 28 microseconds,
and a message takes 113 ps plus 4.4 us for each byte of
data. A node can transmit one packet of data when it
receives the token. A packet contains up to 253 data
bytes, preceded by source and destination identifications
and followed by cyclic-redundancy-check bytes.

Ethernet data links have no token-passing intervals
and theoretically may operate at data rates to 10 Mb/s.
Also, the CSMA/CD protocol allows any node to transmit
whenever the network is free. However, it also allows an
unlimited number of collisions and retries, which makes
the delay indeterminate, a disadvantage in real-time ap-
plications. Ethernet systems can utilize only a small frac-
tion of the theoretical traffic capacity because of the need
to hold down the number of transmission collisions and,
thus, keep the number and retries from multiplying.

A token-passing network utilizes traffic capacity effi-
ciently because time is not spent in resolving those access
contentions. As important for low-cost designs, the inter-
face costs much less because the logic is simpler and the
data rate is moderate. CSMA/CD requires considerable
logic to detect collisions, back off, and try again. Also,
since token-passing keeps the network contention-free,
with only a single node transmitting at a time and in one
direction, signal amplification is unidirectional rather
than bidirectional. This feature contributes to the low
connection cost of PLAN.

The PLAN system is a multicluster network. For in-
stance, the star-like clusters in Fig. 1 could each be in a
separate business department within a single building or
on a corporate campus. Arcnet specifications allow a
segment (basic network) to extend about four miles

PHYSICAL
NETWORK

LOGICAL NETWORK

WORK STATION AND ADDRESS
LINE-ISOLATION DEVICE

4. Logical ring. Token-passing sequence converts the star-burst
topology into a logical ring that makes the physical connections easier
to organize. Additional work stations can be connected to any of the
line-isolation devices to extend the logical ring.

(20,000 feet) and contain 255 nodes. PLAN networks can
be further extended by gateway servers—personal com-
puters that host an internetworking program.

Token passing makes it easy to organize the work
stations and servers. It creates a logical ring within the
star-burst physical topology (Fig. 4). A logical ring’s
physical topology is effectively arbitrary—that is, the
token is simply passed to the next higher address, wher-
ever that station is physically located on the network.
New stations may be added anywhere without reorganiz-
ing addresses. A physical ring, with transmission se-
quence determined by actual node locations, would make
it difficult or inefficient to extend a network over several
floors of a building, for example.

In the PLAN system, network-interface cards in each
work station or server take the place of Arcnet resource-
interface modules, and line-isolation devices correspond
to the main Arcnet cable-connection points (hubs). Each
cluster (star) is formed by connecting RG-62 cable be-
tween the interface cards and the ports of the associated
isolation unit. The clusters are then interconnected by
RG-62 runs between isolation units.

These line-isolation devices may have 10, 20, or 30
connection ports. They condition signals being transmit-
ted from a node to the rest of the network, terminate the
cables, and include transformers to isolate electrical noise
at each end of a cable run. Cables are not tapped be-
tween the terminations, so that signal noise cannot be
generated by multiple signal reflections. Nor can a noisy
node disrupt the overall network because the isolation
units fully buffer the nodes.

Most network communications will revolve around the

new file server and its resident file-processing and man-
agement programs. Cluster/One file servers are dedicated
Apple II 64K models with peripheral disk and tape
drives. They upgrade to PLAN file servers but have only
8-bit computer performance. The PLAN file server is a
proprietary design that allows shared storage to grow to
thousands of megabytes. Also, it can match the perfor-
mance of 8-, 16-, and 32-bit computers.

Based on an 8-megahertz MC68000 with 256-K bytes
of random-access memory, the new file server buffers and
controls up to four 137-megabyte (formatted) hard-disk
drives and four 20- or 45-megabyte streaming tape-car-
tridge backup drives. One of each of these units is inte-
grated with the processor and memory in a cabinet.
Three expansion units bring the total disk capacity to
more than 500 megabytes. Two or more of these con-
glomerations can be connected to each network so that it
can reach up to gigabyte capacities.

Making connections

The server contains a 30-port Ime-isolation device as
well as the standard interface for net-wide communica-
tions. The isolation device directly connects the server to
as many 29 work stations, other servers, or remote isola-
tion units. Besides facilitating network start-up, this ar-
rangement makes it possible to create special sub-
networks with directly connected equipment.

This resident file-management program employs a vari-
able-size, logical volume for efficient use of storage and
handles backup, password protection, error checking,
and retransmission. Other system programs are support-
ed on the personal-computer operating systems: PC-DOS
1.1 and the UCSD p-System for the IBM Personal Comput-
er; DOS 3.2.1, Apple Pascal 1.1, and CP/M 2.2 for the
Apple II; and Apple II emulator mode and SOS 1.1
native mode for the Apple III.

The various network programs are integrated so they
can be employed as a system for such application services
as electronic mail. With the file transfer server, work
stations can send files or entire volumes to remote PLAN
networks, Cluster/One nets, or stand-alone personal
computers. The transfer program supports standard and
automatic-dialing modems and can record all linking and
transfer operations. The Messenger (electronic mail) ser-
vice also handles related functions, like logging the time
of day, scheduling, filing, sorting, printing, and unattend-
ed operation. The print server produces hard copy on a
priority basis or overnight.

The IBM-3270 and -3780 terminal emulators support
terminal-to-terminal communications and remote job en-
try, for transactions with mainframe data bases and
batch processing. Mcreover, the internetworking gateway
server supports real-time communications between differ-
ent segments of extended PLAN networks. To connect a
Cluster/One network to a PLAN net requires a gateway
server: an Apple computer containing both network in-
terfaces and the gateway-server program. Likewise, gate-
ways to other networks will require suitable hardware
interfaces, such as an Ethernet controller, and software
translation of any non-XNS protocols. However, the net-
work and transport software library should simplify
translator development.

NESTAR Systems Incorporated/2585 East Bayshore Road/Palo Alto, CA 94303/ (415) 493-2233
Reprinted from ELECTRONICS, December 29, 1982. Copyright 1982 McGraw-Hill, Inc. All rights reserved.

G500- 1011-0